A large number of carboxylic esters derived from allylic alcohols have been mentioned in the literature as useful intermediates for the preparation of compounds which are of interest in the perfume and flavor industry, as well as for a variety of specialities in the pharmaceutical industry.
Now, such esters have been obtained in the past, with a mixed degree of success, by oxidation of the corresponding olefins.
Various methods have been devised to this end. However, none of these methods is entirely satisfactory.
Thus, when the oxidation of said olefins is carried out for example by air autoxidation, it leads to poorly selective reactions, the intermediate primary hydroperoxide decomposing to provide a mixture of enols and enones. Furthermore, this type of reaction requires large volume reactors and recycling of the starting olefin.
Another method, which consists in the allylic oxidation of the olefins by the action of selenium dioxide, is likewise poorly selective and requires the use of considerable amounts of SeO.sub.2, a toxic and environmentally harmful reagent.
Yet another known method of allylic oxidation resorts to the use of palladium catalysts. For example, allyl acetate, a useful intermediate for the preparation of allyl alcohol, can be obtained by acetoxylation of propene in the presence of palladium. The same applies to the industrial production of 1,4-diacetoxy-but-2-ene.
In this type of reaction, one generally uses a palladium catalyst, preferably on a solid carrier or support such as for example silica or alumina doped with alkaline acetates and/or gold, iron or bismuth (allyl acetate) salts, or with tellurium oxide (1,4-diacetoxy-butene) [see H. Mimoun in "Comprehensive Coordination Chemistry", Ed. Wilkinson, vol. 6, p. 318 (1983) and references therein]. This type of process has not turned out to be very satisfactory either, because, in practice, the conversion does not proceed to completion.
The situation is even more problematic when using superior olefins.
All of the above-described systems use molecular oxygen as oxidizing agent. When employing as catalyst a palladium salt, i.e. in the form of an acetate, the latter is used in relatively high amounts, generally above 2% by weight with regard to the starting product. The reaction is also carried out in the presence of various palladium re-oxidizing agents, such as for example the alkaline nitrates [Tetrah. Lett. 1993, 34, 2523], or benzoquinone [J. Am. Chem. Soc. 1990, 112, 5160; Acta Chem. Scand. 1993, 47, 506; J. Org. Chem. 1991, 56, 5808]together with manganese dioxide, in stoichiometric concentration [J. Org. Chem 1990, 55, 975], or yet of co-catalysts which make it possible to re-oxidize the formed hydroquinone into benzoquinone, by means of the molecular oxygen.
As we have seen, these prior art methods use complex, costly and unprofitable redox systems, in which the recovery and recycling of the precious metal poses difficult technical problems.
We have now discovered that it is possible to carry out reactions of allylic acyloxylation of olefins by reacting the latter with hydrogen peroxide, in the presence of a catalyst consisting of a palladium salt or complex and in the presence of a carboxylic acid.